1. Field of the Invention
The present invention relates to a belt driving apparatus, and more particularly to a belt driving apparatus incorporated in a copier, printer, or the like.
2. Description of Related Art
In general, a belt driving apparatus incorporated in a copier, printer, or the like is faced with a problem in which a dimension error or fitting error in a roller causes a belt to meander in the roller axial direction, that is, in the lateral direction. As a solution to this problem, for example in the belt driving apparatus proposed in JP-A-H8-119484, in each end part of a roller, a C-shaped meandering prevention member having a cut formed in the direction of the outer circumference of the roller is fitted to restrict the meandering of a belt in the roller axial direction.
However, since a belt is formed of a thin elastic material that is highly flexible, when a meandering prevention member is fitted in each end part of a roller, a dimension error in the outer diameter of the roller or in the inner diameter of the meandering prevention member may produce a gap in which the belt may be caught. Letting the belt, thus caught between the roller and the meandering prevention member, rotate may break the edge of the belt.
To overcome this problem, as shown in FIGS. 17A and 17B, a roller 104 is commonly formed such that, of the outer circumference thereof, a middle part 104d, where a belt 101 is hung, is given a different outer diameter from a shaft part 104c at each end, where a meandering prevention member 111 is fitted; that is, the middle part 104d is given a larger diameter than the shaft part 104c at each end to produce a diameter difference in the outer circumference of the roller so that the belt 101 makes contact with the side face of the meandering prevention member 111. This prevents the belt 101 from being caught in the gap between the roller 104 and the meandering prevention member 111. In FIGS. 17A and 17B, reference numeral 105 represents a heater lamp, reference numeral 114 represents a bracket, reference numeral 115 represents a side plate of a belt fixing unit, and reference numeral 116 represents a stop ring.
A diameter difference in a roller as described above can be formed, for example, by swaging each end part of a thin-walled cylindrical metal core, or by fitting, to each end of a metal core, a separately prepared smaller-diameter shaft member. These processes, however, are troublesome and costly, and therefore it is more practical to form a diameter difference by cutting, in each end part of a thick-walled cylindrical metal core, the outer circumference thereof into a predetermined depth. Thus, to obtain sufficient mechanical strength at the shaft part at each end of a roller and simultaneously obtain a sufficient diameter difference, it is necessary to use a thick-walled metal core. This inevitably makes the roller 104 quite thick in the middle part 104d thereof.
Thus, when this belt driving apparatus is used as a belt fixing unit as shown in FIGS. 17A and 17B (here, it is assumed that a bar-shaped heater lamp 105 is arranged in the hollow interior of the roller 104), the roller itself has too high a heat capacity to sufficiently conduct fixing heat to the belt.
In view of the conventionally encountered problem discussed above, it is an object of the present invention to provide a belt driving apparatus that prevents an increase in the heat capacity of a roller and that prevents the belt from being caught in a gap between the roller and a meandering prevention member.